Combustion engines produce exhaust gases that contain pollutants, such as carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions standards for pollutants in an exhaust gas produced by a combustion engine, particularly for vehicular engines, are becoming increasingly stringent. There is a need to provide improved catalysts and exhaust systems for treating and removing the pollutants in such exhaust gases that are able to meet these standards and which are cost-effective.
The exhaust gas from gasoline and diesel engines is commonly treated with a catalyst that is able to oxidise (i) carbon monoxide (CO) to carbon dioxide (CO2) and (ii) hydrocarbons (HCs) to water (H2O) and carbon dioxide (CO2). Three-way catalysts (TWCs) are typically employed to treat the exhaust gas from a gasoline engine, which is able to reduce oxides of nitrogen (NOx) to nitrogen (N2), water (H2O) and carbon dioxide (CO2), in addition to performing oxidation reactions (i) and (ii). The exhaust gas from a compression ignition engine, such as a diesel engine, is typically treated with an oxidation catalyst (commonly called a diesel oxidation catalyst (DOC)) that performs oxidation reactions (i) and (ii). Some diesel oxidation catalysts are also able to oxidise nitrogen monoxide (NO) to nitrogen dioxide (NO2), which can aid removal of NOx by an additional, downstream emissions control device.
Oxidation catalysts for combustion engines typically contain one or more noble metals. The specific noble metal(s) selected for inclusion in an oxidation catalyst will depend on a variety of factors, such as reactivity toward specific pollutants and under differing exhaust gas conditions, cost, durability at high temperatures, chemical compatibility with the support material and any other components of the catalyst, and susceptibility to poisoning by impurities. For example, platinum (Pt) and palladium (Pd) are each able to oxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine. Palladium is more susceptible to poisoning by sulfur in fuel compared to platinum, but is generally cheaper than platinum and has greater thermal durability.
Due to the difference in properties between noble metals, diesel oxidation catalysts have been developed that contain layered arrangements of different noble metals. In such arrangements, the noble metals have been arranged to optimise their combined reactivity and to minimise degradation. For example, WO 2006/056811 describes a diesel oxidation catalyst where the top layer contains palladium and the bottom layer in contact with the substrate contains platinum. Palladium has a lower CO “light-off” temperature than platinum. Such an arrangement facilitates contact between palladium and CO in the exhaust gas, and the heat that is generated can bring the platinum in the bottom layer up to its “light-off” temperature.
An alternative approach is to vary the distribution of the noble metal within the channels of the substrate. U.S. Pat. No. 5,043,311 describes a monolithic or honeycomb-catalyst composed of an inert ceramic or metallic substrate traversed by parallel flow channels, a layer disposed thereon and composed of catalysis-promoting metal oxide and a catalytically active component supported by the metal oxide, wherein the concentration of the catalysis-promoting metal oxide or the catalytically active component increases from the periphery to the central axis of the substrate. The layer composed of the catalysis-promoting metal oxide and the catalytically active component that is disposed in each channel has a uniform composition. However, the composition of each layer varies from channel to channel.
Methods for preparing oxidation catalysts having layered arrangements or for preparing catalysts such as those described in U.S. Pat. No. 5,043,311 have increased production cost, particularly when compared to oxidation catalysts having a single layer where the composition of each layer is the same throughout all of the channels of the substrate. Also, the method employed for preparing multi-layered oxidation catalysts is necessarily more complicated and increases the likelihood of errors in production.
It is therefore desirable to provide an oxidation catalyst arrangement that offers the benefits of a multi-layered catalyst arrangement, but without the increased production cost or difficulty.